[PYTHON] Let Code Day75 starting from scratch "15.3 Sum"

Overview

It seems that coding tests are conducted overseas in interviews with engineers, and in many cases, the main thing is to implement specific functions and classes according to the theme.

Apparently, many engineers take measures on the site called LetCode.

It is a site that trains the algorithmic power that can withstand the coding test that is being done in the early story, and it is an inevitable path for those who want to build a career at an overseas tech company.

I wrote it in a big way, but I have no plans to have such an interview at the moment.

However, as an IT engineer, it would be better to have the same level of algorithm power as a person, so I would like to solve the problem irregularly and write down the method I thought at that time as a memo.

Leetcode

I'm solving it with Python3.

Leet Code Table of Contents Starting from Zero

Last time Leet Code Day74 starting from zero "12. Integer to Roman"

Right now, I'm prioritizing the Medium of the Top 100 Liked Questions. I solved all Easy, so if you are interested, please go to the table of contents.

Twitter I'm doing it.

** Technical Blog Started! !! ** ** I think the technology will write about LetCode, Django, Nuxt, and so on. ** This is faster to update **, so please bookmark it!

problem

15. 3Sum The difficulty level is Medium.

The problem is, if you have an array of n integers nums, then you have an algorithm to check if nums has elements ʻa, b, c such that ʻa + b + c = 0. Design and find all the unique triplets in an array that gives the sum of 0s.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is: [ [-1, 0, 1], [-1, -1, 2] ]

solution

You can choose 3 and choose the combination that will be 0. It may be said that it is an iron rule of combination, but I think it is better to decide what to fix first.

This time I wrote as follows.

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        nums.sort()
        ans = []
        for i in range(len(nums)-2):
            if i > 0 and nums[i] == nums[i-1]:
                continue
            left = i+1
            right = len(nums)-1
            
            while left < right:
                sums = nums[i]+nums[left]+nums[right]
                if sums < 0:
                    left += 1
                elif sums > 0:
                    right -= 1
                else:
                    ans.append([nums[i],nums[left],nums[right]])
                    while left < right and nums[left]==nums[left+1]:
                        left += 1
                    while left > right and nums[right]==nums[right-1]:
                        right -= 1
                    left += 1
                    right -= 1  
        return ans
# Runtime: 1092 ms, faster than 45.79% of Python3 online submissions for 3Sum.
# Memory Usage: 17.1 MB, less than 76.15% of Python3 online submissions for 3Sum.

Sort in ascending order by sorting first. This makes it easier to specify the size of the element when turning the element with a for statement from before.

To give a concrete example, [-1,0,1,2,-1,-4] When sorted, [-4,-1,-1,0,1,2] It will be like that.

The core of this problem is how to select the remaining two elements of 3sum when licking the elements of this from the beginning.

As I wrote in the code, I set ʻi + 1toleft, len (nums) -1toright, and if they were added, their total value, sums`, would be 0. If it is smaller, the left element is +1. If it is larger than 0, right is -1. This is possible because of the sorting mentioned above.

actually -4+(-1)+2=-3<0 So move the left element and -4+(-1)+2=-3<0 This is still the same, so move the left element again -4+0+2=-2<0 It is a flow like that.

And if other than those, that is, sums == 0, the element is added to the prepared ʻans`.

I feel that it is easy to solve such a combination system of sums in a format like this one, so I would like to remember it as a typical problem.

So that's it for this time. Thank you for your hard work.

Recommended Posts

Let Code Day75 starting from scratch "15.3 Sum"
Let Code Day 39 "494. Target Sum" starting from scratch
Let Code Day 33 "1. Two Sum" starting from scratch
Let Code Day8 starting from scratch "1302. Deepest Leaves Sum"
Let Code Day 32 "437. Path Sum III" starting from scratch
Let Code Day 29 "46. Permutations" starting from scratch
Let Code Day 65 "560. Subarray Sum Equals K" starting from scratch
Let Code Day4 starting from scratch "938. Range Sum of BST"
Let Code Day 27 "101. Symmetric Tree" starting from scratch
Let Code Day 41 "394. Decode String" starting from scratch
Let Code Day 25 "70. Climbing Stairs" starting from scratch
Let Code Day69 starting from scratch "279. Perfect Squares"
Let Code Day 34 starting from scratch "118. Pascal's Triangle"
Let Code Day85 starting from scratch "6. ZigZag Conversion"
Let Code Day20 starting from scratch "134. Gas Station"
Let Code Day 88 "139. Word Break" starting from scratch
Let Code Day 28 "198. House Robber" starting from scratch
Let Code Day 36 "155. Min Stack" starting from scratch
Let Code Day 17 "169. Majority Element" starting from scratch
Let Code Day 23 "226. Invert Binary Tree" starting from scratch
Let Code Day 22 starting from scratch "141. Linked List Cycle"
Let Code Day 30 starting from scratch "234. Palindrome Linked List"
Let Code Day68 starting from scratch "709. To Lower Case"
Let Code Day 26 starting from scratch "94. Binary Tree Inorder Traversal"
Let Code Day 46 starting from scratch "406. Queue Reconstruction by Height"
Let Code Day 31 starting from scratch "581. Shortest Unsorted Continuous Subarray"
Let Code Day 38 starting from scratch "208. Implement Trie (Prefix Tree)"
Let Code Day3 starting from scratch "1313. Decompress Run-Length Encoded List"
Let Code Day58 Starting from Zero "20. Valid Parentheses"
Let Code Day16 Starting from Zero "344. Reverse String"
Let Code Day49 starting from zero "1323. Maximum 69 Number"
Let Code Day89 "62. Unique Paths" Starting from Zero
Let Code Day 55 "22. Generate Parentheses" Starting from Zero
Let Code Day56 Starting from Zero "5453. Running Sum of 1d Array"
Let Code Day18 starting from zero "53. Maximum Subarray"
Let Code Day 77 starting from scratch "1502. Can Make Arithmetic Progression From Sequence"
Let Code Day 13 "338. Counting Bits" Starting from Zero
Let Code Day 76 starting from scratch "3. Longest Substring Without Repeating Characters"
Let Code Day71 Starting from Zero "1496. Path Crossing"
Let Code Day 61 "7. Reverse Integer" starting from zero
Let Code Day 82 "392. Is Subsequence" Starting from Zero
Let Code Day51 Starting from Zero "647. Palindromic Substrings"
Let Code Day 50 "739. Daily Temperatures" Starting from Zero
Let Code Day 15 "283. Move Zeroes" starting from zero
Let Code Day14 starting from zero "136. Single Number"
Let Code Day 9 "701. Insert into a Binary Search Tree" starting from scratch
Let Code Day 80 "703. Kth Largest Element in a Stream" starting from scratch
Let Code Day 66 "438. Find All Anagrams in a String" starting from scratch
Let Code Day 11 Starting from Zero "1315. Sum of Nodes with Even-Valued Grandparent"
Let Code Day 43 "5. Longest Palindromic Substring" Starting from Zero
Let Code Day74 starting from zero "12. Integer to Roman"
Let Code Day 42 "2. Add Two Numbers" Starting from Zero
Let Code Day57 Starting from Zero "35. Search Insert Position"
Let Code Day47 Starting from Zero "14. Longest Common Prefix"
Let Code Day78 Starting from Zero "206. Reverse Linked List"
Let Code Day 90 starting from scratch "101 11. Capacity To Ship Packages Within D Days"
Let Code Day 44 "543. Diameter of Binary Tree" starting from zero
Let Code Day 64 starting from zero "287. Find the Duplicate Number"
Let Code Day 84 starting from zero "142. Linked List Cycle II"
Let Code Day24 Starting from Zero "21. Merge Two Sorted Lists"
Let Code Day12 Starting from Zero "617. Merge Two Binary Trees"